23.3 A 3nW Fully Integrated Energy Harvester Based on Self-Oscillating Switched-Capacitor DC-DC Converter

Recent advances in low-power circuits have enabled mm-scale wireless systems [1] for wireless sensor networks and implantable devices, among other applications. Energy harvesting is an attractive way to power such systems due to limited energy capacity of batteries at these form factors. However, the same size limitation restricts the amount of harvested power, which can be as low as 10s of nW for mm-scale photovoltaic cells in indoor conditions. Efficient DC-DC up-conversion at such low power levels (for battery charging) is extremely challenging and has not yet been demonstrated. Boost DC-DC converters are widely used to harvest energy from DC sources and yield high conversion efficiency [2]. However, they require a large off-chip inductor at low harvested power levels, increasing system size. Alternatively, switched-capacitor (SC) DC-DC converters can be fully integrated on-chip and are favored for form-factor constrained applications [3-8]. However, at low power levels, SC converter efficiency has been constrained by overheads of clock generation and level-conversion to drive the switch capacitors. As a result